Communication Network with Co-Routed Multi-Channel Traffic

ABSTRACT

Embodiments of the present invention route a wavelength division multiplexed signal across multiple communication paths using skew characteristics of at least some of the communication paths. The network is a wavelength division multiplexed optical transport network. The plurality of communication paths involves different signal and path attributes such as a plurality of carrier wavelengths, optical carrier groups, physical communication paths (different nodes, different fibers along a same path, or any combination of the foregoing), or any other differentiating factors between two paths.

CROSS REFERENCE TO RELATED PATENT APPLICATIONS

This application is related to U.S. Provisional Application Ser. No.60/885,832, entitled “Communication Network with Skew Path Factoring,”filed Jan. 19, 2007 and U.S. application Ser. No. 11/781,912, filed onJul. 23, 2007 entitled “Communication Network with Skew Path Monitoringand Adjustment,” both of which are incorporated herein by reference intheir entirety.

BACKGROUND

A. Technical Field

This invention relates generally to optical transport networks, and moreparticularly to the management of skew across a wave divisionmultiplexed network.

B. Background of the Invention

Optical networks are able to communicate information at high data rates.An optical transport system 10 is shown in FIG. 1, with multipleintermediate nodes and routes 16 between source 12 and destination 14.Nodes n1-n6 in a network are provided as an exemplary network withspatial diversity in the span, or segments separating nodes, e.g.,across a geographic area. Multiple communication paths between a sourcenode and destination node are provided across the network. The transportsystem might consider the route length, the traffic load, the routecost, and latency property, among other factors, for a given signal whenchoosing a path within the network on which to transport the signal. Forexample, a high quality of service (“QoS”) request might require a givensignal be transported on a route between a source and a destination withthe lowest amount of latency. Alternatively, as traffic data ratescontinue to mushroom, carriers typically resort to routing signals onalternative and/or relatively higher latency paths, which often timesspan a longer overall distance than the preferred path. Additionally,these longer paths typically have more nodes, which usually translatesinto compromised timing properties for the signal at the receiver.

SUMMARY OF THE INVENTION

Embodiments of the present invention route a signal as signal portionsover multiple paths in an optical network using skew characteristics ofat least some of the communication paths. The network can be a wavedivision multiplexed (“WDM”) optical transport network using wavelengthdivision multiplexed wavelengths and/or optical carrier groups (“OCGs”)over a fiber link to another node in the network. The plurality ofcommunication paths involves different signal and path attributes suchas a plurality of carrier wavelengths, optical carrier groups, physicalcommunication paths (different nodes, different fibers along a samepath, or any combination of the foregoing), or any other differentiatingfactors between two paths.

In certain embodiments of the invention, communication paths areselected relative to an analysis of skew on one or more of the selectedcommunication paths and corresponding wavelengths. The associatedinformation is routed on a path or paths with a minimum skew so that thesequential arrival of the information at a receiver is improved.Accordingly, the transmission of the associated information on thecommunication path(s) is controlled so that reassembly of theinformation becomes more efficient due to the relative arrival ofportions of the information from a network to the receiver. Thetransmission of the associated information may be done as a virtualsuper wavelength or as a plurality of super wavelength groups.

In some embodiments, latency information is stored and maintained in alook-up table. The latency information can be used to estimate the skewbetween two network paths.

In certain other embodiments of the invention, a path with a preferredminimum skew is not available and an alternate routing is performed. Insome embodiments, the signal may be unable to be co-routed on multiplepaths and a notification is sent indicating that co-routing isunavailable at that time.

In other embodiments, a failure event has occurred and protectionswitching is performed taking into consideration the skew of the pathswitched to as a result of the failure event.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference will be made to embodiments of the invention, examples ofwhich may be illustrated in the accompanying figures. These figures areintended to be illustrative, not limiting. Although the invention isgenerally described in the context of these embodiments, it should beunderstood that it is not intended to limit the scope of the inventionto these particular embodiments.

FIG. 1 is a schematic of an optical transport network, in accordancewith various aspects of the present invention.

FIG. 2 is a block diagram of a communication network that transmits asignal over multiple channels considering skew information for routingthe multiple channels, in accordance with various aspects of the presentinvention.

FIG. 3 is a functional block diagram of a communication network thatconsiders skew information for routing information on a network, inaccordance with various aspects of the present invention.

FIG. 4 is a functional block diagram of a system illustrating a priorilatency determination.

FIG. 5 is a functional block diagram of a system illustrating empiricallatency determination, in accordance with various aspects of the presentinvention.

FIG. 7 is a flowchart of a process to empirically measure the skew ofinformation transmitted by multiple communication paths in acommunication network, in accordance with various aspects of the presentinvention.

FIG. 7A is a network illustration of skew performance variationoccurring over multiple communication paths via multiple routes, and theidentification of the better route in terms of skew, in accordance withvarious aspects of the present invention.

FIG. 7B is a network illustration of skew adjustment via wavelengthreassignment of the multiple communication paths at a node between thesource node and the destination node, in accordance with various aspectsof the present invention.

FIG. 7C is network illustration of skew adjustment via selection of onefiber from multiple fibers depending on the dispersion slope of thefiber, in accordance with various aspects of the present invention.

FIG. 7D is a network illustration of dividing a virtual super wavelengthinto multiple virtual wavelength groups and routing them on differentroutes on the network, in accordance with various aspects of the presentinvention.

FIG. 8 is a schematic of a transceiver node with its internally coupledand switched band modules each coupled to different nodes for spacediversity routing, in accordance with various aspects of the presentinvention.

FIG. 9 is a schematic of a receiver portion of a line module wherein thereceiver has optical skew compensation with electronic skew measurementand buffer, in accordance with various aspects of the present invention.

FIG. 10 is a schematic of a transmitter portion of a line module whereinthe transmitter has optical skew compensation and electronic skewmeasurement and buffer, in accordance with various aspects of thepresent invention.

FIG. 11 is an optical system having both dispersion compensatingelements and skew compensating elements according to various embodimentsof the present invention.

FIG. 12 is an optical receiver system in which dispersion and skewcompensation is performed according to various embodiments of thepresent invention.

FIG. 13 is an optical system comprising both terrestrial and submarineoptical networks according to various embodiments of the presentinvention.

FIG. 14 is a block diagram of a communication network that transmits asignal over multiple channels and performs protection switchingconsidering skew information for routing the multiple channels, inaccordance with various aspects of the present invention.

FIG. 15 is a flow chart illustrating creation and maintenance of latencylook-up table.

FIG. 16 is a flow chart illustrating routing selections consideringskew.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description is set forth for purpose of explanation inorder to provide an understanding of the invention. However, it isapparent that one skilled in the art will recognize that embodiments ofthe present invention, some of which are described below, may beincorporated into a number of different computing systems and devices.The embodiments of the present invention may be present in hardware,software or firmware. Structures shown below in the diagram areillustrative of exemplary embodiments of the invention and are meant toavoid obscuring the invention. Furthermore, connections betweencomponents within the figures are not intended to be limited to directconnections. Rather, data between these components may be modified,re-formatted or otherwise changed by intermediary components.

Reference in the specification to “one embodiment”, “in one embodiment”or “an embodiment” etc. means that a particular feature, structure,characteristic, or function described in connection with the embodimentis included in at least one embodiment of the invention. The appearancesof the phrase “in one embodiment” in various places in the specificationare not necessarily all referring to the same embodiment.

FIG. 2 illustrates a portion of an exemplary networking system wherecommunication bandwidth and Quality of Service (“QoS”) can be increasedby transporting information, such a client signal, over multiplecommunication paths within the network system. Information is any data,such as content, control, overhead, metadata, redundant or protectiondata, correction data, etc. that is transported along a path in thenetwork.

The portion of the networking system shown in FIG. 2 can, in variousembodiments, incorporate portions of legacy communication networks alongwith additional control, methods and/or apparatus to enable themeasurement, control, and/or adjustment of skew on the communicationnetwork as described in the present disclosure. A node in the networkingsystem can be any node where transmitted information is managed,processed and/or signal performance is evaluated via measurementdevices.

In accordance with certain embodiments of the invention, nodes can betraditional analog nodes, digital nodes, hybrid nodes that allow signalmanagement, or any combination thereof. Analog nodes may be amplifiers,or regeneration nodes. Nodes can also be digital nodes, implementing anoptical to electrical to optical translation (“OEO”) such as describedin case as disclosed and taught in U.S. patent application Ser. No.10/267,331, filed Oct. 8, 2003, entitled “TRANSMITTER PHOTONICINTEGRATED CIRCUITS (TxPIC) AND OPTICAL TRANSPORT NETWORKS EMPLOYINGTxPICs” and in U.S. patent application Ser. No. 10/267,212, filed Oct.8, 2002, entitled “DIGITAL OPTICAL NETWORK (‘DON’) ARCHITECTURE”, andU.S. Pat. No. 7,116,851, issued Oct. 3, 2006, entitled “AN OPTICALSIGNAL RECEIVER PHOTONIC INTEGRATED CIRCUIT (RxPIC), AN ASSOCIATEDOPTICAL SIGNAL TRANSMITTER PHOTONIC INTEGRATED CIRCUIT (TxPIC) AND ANOPTICAL TRANSPORT NETWORK UTILIZING THESE CIRCUITS”, all of which patentapplications and patents are incorporated herein by reference. Referenceto measuring signal performance can be implemented in either theelectrical or optical domain.

Information can be transported as a signal or signals. FIG. 2 shows oneexample of communication between node 3210 and node 3220. Network 3240represents any intermediary networking system, including but not limitedto, fiber, intermediary nodes or intermediary networking systems betweennodes 3210 and 3220.

Referring to FIG. 2, a signal routed from node 3210 to 3220 can bedivided into a plurality of signal portions 3230. One method of dividinga wave division multiplexed signal is to demultiplex the signal prior totransmission. This provides more flexibility for the network since eachsignal portion can be routed separately and then restored at thereceiver node 3220. The signal portions can be routed on a plurality ofdifferent channels on a single or multiple communication paths. In oneembodiment, each channel can be a carrier wavelength. The communicationpaths can be paths on a single fiber, paths through common intermediarynodes, paths on different fibers or through different intermediarynodes.

FIG. 2 illustrates the signal divided into four signal portions 3230,however, as understood by one of ordinary skill in the art, the signalcan be divided into two or more signal portions. Once the signal hasbeen divided, the divided signal portions 3230 can be transported asdifferent channels. The wavelengths can be transported as part of agroup called a virtual super wavelength.

Each channel can be transported on a different communication path 3250,3252, 3254, 3256 providing for added flexibility in routing the signals.Consequently, the networking system is not limited to selecting a pathcapable of transporting the entire signal since the signal is dividedinto multiple signal portions that can be transported separately. Thisimproves QoS and permits higher bandwidth signal transportation overlonger distances.

However, since the signal transported was divided prior to transmission,it must be combined at the destination node 3220 to recreate thetransported signal. In order for the original signal to be restored atthe destination node 3220, the skew between the channels 3235 should bewithin a skew constraint. Skew may be defined as a variation relative tothe initial timing of a component of a launched data signal ordifferential latency between the signal portions.

Skew can arise from many different causes depending upon thetransmission medium and length over which information is communicated.For example, intrachannel skew and interchannel skew can arise becausedifferent wavelength carriers propagate at different rates. Inparticular, a high frequency carrier signal will generally take arelatively longer period of time to propagate along an identical lengthfiber as a lower frequency carrier signal. Skew can also arise becausethe different channels are transported on different paths. The paths maybe of varying lengths or have varying numbers of intermediary nodes.Skew becomes an increasingly important consideration when routingsignals on different paths because the skew can grow tremendously as aresult of varying latencies between the paths.

As shown in FIG. 2, separate communication paths 3250, 3252, 3254, and3256 are chosen. In the embodiment shown in FIG. 2, four different pathsare selected. The networking system of the present invention considersskew in selecting the communication paths by determining, monitoring,analyzing, and adjusting the skew, as further described below.

FIG. 3 illustrates a functional block diagram 200 of a communicationnetwork that considers skew data for routing information across anetwork in accordance with various aspects of the present invention.Ingress data 204 is provided to communication network 206 as clientsignals in the data plane 202 and is communicated to another node in thenetwork and exits as client signal egress data 208 in the data plane202. In a communication network using multiple communication paths totransport a signal as signal portions, skew should be addressed in orderto avoid certain types of network failure events such as droppingsignals, losing packets caused by overflowing a memory buffer andmissing quality of service standards.

Embodiments of the present invention provide for route selection andskew adjustment 206A in communication network 206 via controller 226 inthe control plane 220 of network 200. The controller 226 receivestraditional routing factors input 222, such as distance, traffic load,and other factors related to characteristics of the path(s). Controller226 also receives system skew information input 224 that can be derivedfrom theoretical skew data calculation input 224B described below inreference to FIG. 4, or from empirically measured system performance andcharacteristics input 224A described below in reference to FIG. 5.

In some embodiments, controller 226 can also receive monitored skew datainput 224C from monitor module 236. Monitor module 236 monitors the skewin the network at a particular interval and provides real-time monitoredskew data 224C to the controller 226. The monitored skew data may differfrom the skew input received from the empirical skew data 224A and thetheoretical skew data 224B.

System skew 224 may be existing skew properties in the network that mustbe overcome in order to meet an allowable skew specification at adestination. Additionally, controller 226 also receives input data 228for skew correction, such as theoretical and available electronic andoptical skew adjustment resources, re-routing options and availabilityand prioritization of resources, etc.

In some embodiments, controller 226 can also receive input frominitialization module 230 and notification module 232. Initializationmodule 230 performs functions associated with initial determination ofnetwork paths when ingress data 204 is sent through communicationnetwork 206. The initialization module 230 considers skew in selecting acommunication path for the signals transported. In the event no pathscan be selected with acceptable skew, the notification module 232 sendsa notification alerting a user of the system that no path can beselected with acceptable skew.

In some embodiments, protection switching module 242 detects a failurefor example a broken cable or a cable that has sufficient signaldegradation to effectively be broken. The detection could also beaccomplished as part of the skew analysis since a failure will result ina large skew value. Protection switching module 242 also performs aprotection switch. The protection switching can use any protectionswitching technique. However, when the protection switching isperformed, skew is analyzed and considered as described in thisdisclosure.

In one embodiment, the controller utilizes look-up table 234 to maintaindata related to skew. The data related to skew can be stored in the formof latency values associated with each span or multiple spans. By way ofdemonstration, an exemplary version of look-up table 234 is shown below.One skilled in the art will understand that the table shown is only oneexample and that the table can be stored in software, hardware, firmwareor a combination thereof.

The information contained within look-up table 234 may also be stored inanother, non-tabular, format. The table below has two columns: path andlatency. The path column includes information related to variousdifferent paths within the network system that are available. The pathinformation could be related to a communication between two nodes alonga fiber or between two nodes along a fiber with intermediary nodes orsystems. The latency column indicates the latency associated with thepath in the same row. The present invention, including look-up table234, could operate in conjunction with a conventional networking system.FIG. 1 is referenced to describe the paths shown in the table below.

Path Latency n1-n2 t₁  n2-n3 t₂  n1-n4 t₃  n2-n4 t₄  n4-n5 t₅  n5-n6 t₆ n4-n3 t₇  n2-n6 t₈  n5-n3 t₉  n6-n3 t₁₀

The exemplary look-up table above illustrates using latency values foreach span rather than every path in the network. One skilled in that artwill understand the table could be expanded to include latency valuesassociated with additional paths as well as skew data associated withspan or path pairs or groupings.

The latency values in the above look-up table can be determinedempirically resulting in empirical skew data 224A, theoreticallyresulting in theoretical skew data 224B, or dynamically by monitoringthe network system resulting in monitored skew data 224C, as will bedescribed below with reference to FIGS. 4, 5 15, and 16.

The above look-up table can be used to determine the skew between anytwo or more paths on the networking system shown in FIG. 1. For example,to communicate between nodes n1 and n3, there are many available pathssome of which are listed and described below:

1) n1 to n2 to n3

2) n1 to n4 to n3

3) n1 to n2 to n6 to n3

4) n1 to n4 to n5 to n3

5) n1 to n2 to n4 to n5 to n3

6) n1 to n2 to n6 to n5 to n3

7) n1 to n4 to n5 to n6 to n3

8) n1 to n4 to n2 to n6 to n3

In one embodiment, the communication between nodes n1 and n3 can bedivided into two or more signal portions. Each signal portion can betransported on a different path. For example, one portion can betransported on path number 1 (n1 to n2 to n3) and another portion can betransported on path number 8 (n1 to n4 to n2 to n6 to n3). Controller226 can use look-up table 234 to determine the latency associated withpath number 1 and the latency associated with path number 8 by summingthe latencies of the individual spans or by incorporating additionalrows in the table shown above to store the latencies for paths withintermediary nodes. For example, the latency for path number 1 (L₁) isthe sum of t₁ and t₂ and the latency for path number 8 (L₈) is the sumof t₃, t₄, t₈, and t₁₀. Additionally, the skew between path number 1 andpath number 8 can be determined. Alternatively, look-up table 234 can beexpanded to include skew as well as latency. The skew between pathnumber 1 and path number 8 is the absolute value of the differencebetween L₁ and L₈.

Skew can also be determined between three or more paths using a similarequation. For example, skew between path numbers 1, 2, and 8 can besimilarly calculated. The latency associated with path number 2 (L₂) isthe sum of t₃ and t₇. The skew between the three paths is the absolutevalue of the difference between the greatest and least of the latencyvalues. For example, assuming L₁<L₂<L₈, then the skew is the absolutevalue of the difference between L₁ and L₈. A similar skew determinationcan be used for skew between four or more paths.

Initialization module 230 can use the information relating to skew toselect routes or paths for the various signals and signal portions. Forexample, initialization module 230 can compare the skew between pathsnumbers 1 and 8 to a skew threshold. If the skew is less than the skewthreshold, then the information can be co-routed on paths 1 and 8. Ifthe skew is greater than the skew threshold, then other available pathscan be analyzed based on skew using a similar procedure to the onedescribed above. If there are paths that can be selected, then theinitialization module 230 and the route selection/skew adjustment module206A selects the paths that meet the skew constraints. Otherwise,notification module 232 sends an alert indicating that the signal cannotbe divided and transported. Additionally, each possible path can beexamined and the paths with the least skew can be chosen. Furthermore,skew can be included along with a number of other considerations used inselecting an optimal set of paths. The other considerations may bebandwidth considerations, traffic load, Quality of Service, routelength, latency, and any other relevant consideration.

One way to determine the skew threshold or skew constraint is toevaluate the amount of skew that can be compensated for or adjusted bythe networking system, as described below in reference to FIGS. 7A-D and8-12. There are a number of ways to compensate for skew or adjust skew,including, compensating for skew at the transmission node, at thereceiver node, or at any or all intermediary nodes. Skew compensationcan be achieved in the optical domain using one or more optical buffers,coils of fiber. Skew compensation can also be achieved in the electricaldomain using one or more first-in-first-out (“FIFO”) buffers. The sizeof the optical and electrical buffers can be adjusted thus altering theskew constraints.

The modules and functionality shown in the control plane of FIG. 3 canbe accomplished centrally within the network, at each node in thenetwork, or a combination thereof. A central network management stationcan include a controller, or microprocessor, with associated memory,I/O, and other hardware/software to enable the execution of logicalinstructions on input and output data. The network management stationcan be a UNIX box, or any other operating system suitable to accomplishthe functions mentioned herein. Central network management station islinked to the nodes of the network system.

The present invention is well suited to any coupling arrangement, viaany medium, to allow communication between the data and control planesin communication networks. The present invention may only link a portionof the nodes in parallel, which then could subsequently link a coupledseries of nodes.

Alternatively, a distributed network management architecture could beemployed. In particular, at least one node could have connectivity toanother node (intranodal) to allow for the communication of resourcestatus in the node for skew adjustment. The present invention is wellsuited to any form of connectivity that allows for distributed controlfor skew measurement, communication, status, control, and/or etc.to/from a node, e.g., by optical supervisory channel (“OSC”). A givengateway network element (“GNE”) might have connectivity to multipleservice network elements (“SNEs”).

Alternatively, each node may have standalone skew measurement andcorrection capacities to simplify the required interaction between thenodes. The present invention is well suited to any combination of theseor other control models that allow skew measurement and/or adjustment.

FIG. 4 is a block diagram further illustrating a priori determination oflatency used to generate the theoretical skew data 224B shown in FIG. 3.In the embodiment shown in FIG. 4 latency is predicted in advance ofcommunication on the networking system. The latency differential betweentwo communication paths is used to determine the theoretical skew data224B as described above. Since communication paths utilize one or morefibers and nodes and latency information about the fibers, fiber length,and nodes can be known, that information can be used to predict thetotal span latency. Fiber information 3410 and node information 3420 areinputs into latency determination module 3430. Latency determinationmodule 3430 outputs latency as result of a fiber 3440 and latency as aresult of a node 3450. The fiber latencies 3440 and node latencies 3450are summed at summing junction 3460 to compute the span latency 3470.The fiber information 3410 used by latency determination module 3430includes properties of the fiber related to the latency that resultsfrom transmission along the fiber, for example, the length anddispersion of the fiber. Node information 3420 includes properties ofthe node related to the latency within a node. Once span latency 3460 isdetermined, look-up table 234 or other latency reference can be updatedto include the predicted latency values 3470.

Another way to determine latency, and therefore skew, is to determine itempirically by measuring the skew between two paths. Skew can bedetermined empirically prior to setting up a communication path. FIG. 5is a block diagram illustrating an empirical determination of latencyacross a span. Referring to FIG. 5 a network span is shown betweentransmission node 3510 and receiver node 3525. In the embodiment shownin FIG. 5 the signal is divided into two portions 3515 and 3520 forcommunication. A timing marker is included as part of each signal. Themarker is a preset bit pattern that will be used by a receiver node 3525to match the same pattern in a test signal, whose arrival time will thenbe evaluated. The marker should be sufficiently unique, such as apseudo-random binary sequence (“PRBS”), that it is not confused with adata signal. The marker may be a stand-alone signal(s) or may beinserted into a client signal running live traffic.

When the signal is received at receiver node 3525 a set offirst-in-first-out (“FIFO”) buffers 3530 and 3535 can be used to storethe signal, including the timing marker. The time difference between thetiming markers indicates the skew between the two communication pathsshown in FIG. 5. One skilled in the art will recognize that the aboveempirical skew determination can be used with two or more twocommunication paths.

In one embodiment, both theoretical and empirical skew determinationsmay be employed. Look-up table 234 may be created and/or updated basedon theoretical skew data 224A, empirical skew data 224B, or acombination thereof. Monitor module 236 determines monitored skew data224C by repeating the empirical skew determination described above atregular intervals. Monitored skew data 224C can be used to dynamicallyupdate look-up table 234.

The flowchart in FIG. 6 describes a method, independent of structure,which may be implemented in various embodiments of the invention. Invarious embodiments of the invention, associated data from a clientsignal is distributed and routed by different physical routing (nodesand fibers), different wavelength groupings, different wavelengthsand/or Optical Carrier Groups (“OCGs”), and with different skewadjustments. The specific communication paths provided therein are anexemplary allocation of routing and skew adjustments by a controllerthat evaluated system skew performance and resources.

FIG. 6 is a flowchart 1700 of a process to empirically measure the skewof information transmitted by multiple paths in a communication networkin accordance with various aspects of the present invention. Measuringthe skew of a given path can occur prior to establishing a circuit for agiven client signal to provide more reliability in the quality of thesignal as it is transported through a network.

A communication path(s) is selected 1704 in order to test skewproperties thereof. Communication paths may be defined as having variouslengths with differing number of intermediary nodes including, but notlimited to, span-wise evaluation, route-wise evaluation (e.g., fromsource node to destination node), or round-trip-wise and then back tooriginal source node).

A marker is generated 1706 for transmission on the chosen communicationpath(s). The marker is transmitted 1710 on multiple communication pathsin the network. The communication paths can be tested in a parallelfashion, such that relative skew between two communication paths may bemeasured, or tested in series with synchronization and timingcomparisons made by comparison to an accurate reference clock. Thetransmission of the test signal with marker can be performed eitherwhile the entire network is down, or while the network is communicatingtraffic on channels other than the channels, or communication paths, tobe tested.

The skew is measured 1712 and output as 1712A skew performance andcommunicated to either local nodes or to centralized controller. Skewdata can be stored as a new variable, or object, in the Link StatedAdvisory (“LSA”) table, for consideration in choosing a communicationpath in the network.

If diversity of communication paths exists 1720, in terms of carrierwavelengths, OCG groups, physical routing on nodes or fibers, etc., thenin step 1720A, a new route is selected and is evaluated using the markerat step 1706 onward. In this manner, the combinations and permutationsof communication paths available in the communication network can betested and evaluated for future use. The test process 1700 can berepeated at timely intervals, such as programmed maintenance (“PM”),existing downtime, or as interleaved with revenue traffic on thenetwork, as resources permit, especially during low traffic periods. Ifan update provides a substantial change in the skew performance, noticesor interrupt signals may be generated and forwarded to appropriateusers, controllers, for remedial management of the network. In oneembodiment, the networking system not only determines the skew, but canalso compensate for the skew.

FIGS. 7A through 7D illustrate different skew routing and skew adjustingtechniques that allow for improved efficiency, flexibility, andcost-effective skew management of information transmission through thenetwork. An exemplary client signal to be transmitted on the networks400A through 400D is demultiplexed into multiple channels to accommodatethe size of the client signal. In this case, client signal is brokeninto multiple signal portions C₁-C₄ and communicated on network carrierfrequencies, shown as λ₁, λ_(K), λ_(L), and λ_(M), which are alsoreferred to as a first virtual super wavelength (“VSW1A”).

The quantity and frequency of channels within an OCG may vary inaccordance with the network system and environment in which it operates.For example, an OCG may depend on the resources available on thenetwork, the skew and traditional metric performances of the network,and the controller assigning the resources. The VSW1A is received andre-sequenced at the destination node (e.g. node N3) with acceptable skewperformance, for reconstruction of the client signal and egress from thenetwork. The information routing or skew adjustment described herein canbe employed in combination or permutation with each other to provideadditional options in routing and skew adjustment for the overallsystem. FIGS. 7A through 7D can be implemented in one embodimentemploying hardware shown in FIGS. 8-10, and employing processesdescribed in FIGS. 15 and 16.

Referring to FIG. 7A, a network illustration 400A of skew performancevariation occurring over multiple communication paths and theidentification of the better route in terms of skew is shown, inaccordance with various aspects of the present invention. Initial timingof associated data can either be synchronized as shown in initial timing402 at to, or can implement a preskew timing of signals 402A, whereininformation on λ₁ is delayed relative to the other wavelengths. Thegroup of wavelengths is increasingly delayed from λ₁ to λ_(M) of VSW1Arelative to with a preskew dispersion slope K₀.

The allocation of data across the multiple routes is determined by theskew between the channels (e.g., λ₁, λ_(K), λ_(L), λ_(M)). If associateddata signals are transmitted on route P₁ 410, then the resulting skew isthe time difference between the earliest signal t_(E) and latest signalt_(L) occurring between the signals at their destination, node N3,illustrated as skew 404 (e.g., time t_(s1)) with an associated skewdispersion slope of K₁. Alternatively, if the associated data signalsare transmitted via route P₂ at 412, there results at the destinationnode N3 a timing skew 406 is illustrated, such as t_(S2), with anassociated skew dispersion slope of K₂.

The skew associated with the different routes P₁ and P₂ may be analyzedat the destination node to select an optimal route. These differentskews may also be compared to certain parameters 408, such as maxallowable skew t_(MAX), or maximum allowable skew slope K_(MAX) in orderto select a preferred route. The skew may also be analyzed atintermediate nodes to select an optimal route or identify that skewfalls within parameters.

The evaluation of skew may identify that skew has fallen outside of apreferred specification or range, and initiate a skew adjustingprocedure. The skew consideration of each link, or span, in the networkmay be considered and summed for analysis relative to the allowable skewtolerance for a given communication network specification or standard.

Referring to FIG. 7B, a network illustration 400B of skew adjustment viawavelength reassignment of the multiple communication paths is shown inaccordance with various aspects of the present disclosure. In theseembodiments, a given client data signal is separated into foursubsignals (e.g., signal portions C₁-C₄) to be routed on differentcommunication paths over the network as associated data. Associated datameans the multiple signals are associated with each other as being partof the original client data signal and are reassembled at a destinationnode to recreate the client data signal.

The format of the signal portions may depend upon the protocol of agiven system such as protocols defining the distribution of payload,forward error correction (“FEC”) data, overhead (OH) data, etc. Assuminginitial timing 402 in FIG. 7B, if a set of associated data signals aredetermined to have unacceptable skew performance at the destination orany intermediate node, wavelength reassignment may be utilized toimprove the skew performance in the nodal network.

At an intermediate node, for example, if the signal iswavelength-swapped, then interchannel dispersion occurring between highand low frequencies can be compensated by inversing the wavelengthswhere the longest wavelength is swapped for the shortest transmissionwavelength and the next longer wavelength is swapped for a shorterwavelength. In effect, the wavelengths are reversed in a manner thatpreviously longer wavelength signals are substituted with shorterwavelength signals. For example, signal portion C₃ and C₂ are reroutedto be carried on swapped frequencies (e.g., C₃ is now carried on λ_(K)and C2 is now carried on λ_(L)). This can be accomplished by opticalsignal wavelength conversion, or by an optical-to-electrical-to-opticalconversion that reassigns a signal portion to be transmitted on achannel with a different frequency laser.

If associated data signals are received at node N2 with dispersion slopeK₃, as shown in the upper left side of FIG. 7B at 417, then the carrierwavelengths can be wavelength-swapped for a given set of associated datafor a given client signal. Thus, if signal portions C₁-C₄ of a clientsignal are transmitted on carriers λ₁, λ_(K), λ_(L), and λ_(M),respectively at source node N1, they can be transposed at intermediatenode N2 to carriers λ_(M), λ_(L), λ_(K), and λ₁, respectively, with newdispersion slope K^(T) ₃ as seen at 420 in FIG. 7A. Wavelengthreassignment in this embodiment assumes a linear dispersion slope ofsignal portions C₁-C₄ on carriers λ₁, λ_(K), λ_(L), and λ_(M).

In an alternative embodiment, any signal portion can be reassigned toany carrier frequency, as best fits the overall skew reduction for thesystem, e.g., for non-linear channel performance as illustrated at 422in FIG. 7A. After performing the wavelength reassignment, associateddata C₁-C₄ is received at destination node N3 with a resultant adjusted,or minimized, skew 422 of t_(S3), and associated nonlinear dispersionslope of K₄; a superior skew performance than the same signals wouldhave had without the wavelength reassignment. Minimal skew is theresultant skew of the client signal portions at the destination nodethat meets the specified allowable skew tolerance for the system andthat has been managed by the controller to provide either the leastamount of skew available for the VSW or VWG on the available resourcesof the network or with a reasonable amount of skew in consideration forother performance tradeoffs.

Referring now to FIG. 7C, a network illustration 400C illustrates skewadjustment via selection of one fiber from a possible group of differentmultiple fibers depending at least in part on the particular dispersionslope, K_(X), of the fiber which is shown in accordance with variousaspects of the present invention. Multiple optical fiber links, F₁ 440to F_(N) 444, are coupled between nodes N6 and N3, where each fiber mayhave different dispersion compensation slopes, K₅ and K₆, respectively.The multiple fiber route scenario via F₁ 440 to F_(N) 444 is similar tomultiple physical routes 410 and 412 involving different nodes in thenetwork as illustrated in FIG. 7A. However in the present embodiment,there may not essentially be any diversity in fiber lengths of the fibergroup between nodes N3 and N6 since these two nodes are at the samedistance apart for any one fiber of the fiber group F₁ 440 to F_(N) 444.Thus, a finer skew adjustment may be possible by considering only thenonlinear variations of the different fibers.

Referring now to FIG. 7D, a network diagram illustrates a divided clientsignal into two routes in accordance with various aspects of the presentinvention. As defined above, a client signal may be routed as a virtualsuper wavelength (“VSW”); for example, the client signal is co-routed onmultiple channels on the same path, the same nodes and/or fiber. If aVSW routing is not available, then the client signal may be routed alongdifferent routes on the network as two or more virtual wavelength groups(“VWG”) (e.g. routed as multiple groups wherein each group of one ormore channels is routed on the same path).

A VWG can be any size and grouping of signals as is appropriate forchannel bandwidth between nodes, and that skew and other performancespecifications allow. In the present example, associated data, VSW, isinitially scheduled to be transmitted as associated client signalportions C₁-C₄ on carriers λ₁, λ_(K), λ_(L), and λ_(M), where clientsignal portions C₁-C₄ refer to a portion of the client signal that istransmitted on any available carrier, e.g., λ₁, λ_(K), λ_(L), and λ_(M).The specific content of C₁-C₄ and the specific wavelengths on any givenpath are decided by the controller, such as a central controller 302 ora node controller. Thus, as the traffic rate increases, the contentdistribution C₁-C₄ may vary across the respective carriers, e.g., λ₁,λ_(K), λ_(L), and λ_(M). In fact, if the controller so evaluates it, theclient signal may be adjusted from content distribution C₁-C₄ oncarriers, e.g., λ₁, λ_(K), λ_(L), and λ_(M) to content distributionC₁-C₃ on respective carriers, e.g. λ₁, λ_(K), λ_(L), and λ_(M).

However, in this illustration, sufficient channel count, or bandwidth,was not available on path P₃ 470 between the source node N1 and thedestination node N3 to co-route the entire client signal (e.g., clientsignal portions C₁-C₄) as a Virtual Super Wavelength, VSW1B 464.Consequently, the exemplary controller, evaluate the network demands(e.g., traffic load, network resources, bandwidths, etc.) and concludethat the VSW should be divided into two or more virtual wavelengthgroups. For example, VWG1 may be divided into client signal portions C₁and C₄ on carriers λ₂ and λ₃₁ on transmitted on path P₃ 470, andwavelength group VWG2 may be divided into client signal portions C₂ andC₃ on carriers λ₄ and λ₅ on path P₄ 472. For simplicity, it is assumedthat carrier wavelengths are consistent across the several spans shown,though carrier wavelength diversity can be used.

Note that in the present embodiment, client signal portions C₁ and C₄are co-routed as one VWG2 on outer wavelengths λ₂ and λ₃₁, while clientsignal portions C₁ and C₄ are co-routed as another VWG3 on nominalwavelengths λ₄ and λ₅, similar to that illustrated in prior FIG. 7B. Inthis manner, skew of VWG2 at node N2 466 may undergo skew adjustmentprocedure because the more extreme frequency values, λ₂ and λ₃₁ of VWG2will exhibit more skew, K₇, at node N2, than the nominal frequencyvalues λ₂ and λ₃ of VWG3 with skew K₈ illustrated at N5 468. Thus,client signal portions C2 and C3 may not require skew adjustment betweenthe source node and destination node.

Different quality of service signals may be routed in this manner toprovide preferred performance characteristics. If client signal portionsC2 and C3 are more time-sensitive, or contain more sensitive data, theportions may be transmitted on a preferred physical route, preferredcarrier wavelength, preferred grouping, and/or preferred fiber (i.e.,preferred with respect to minimized skew slope, signal dispersion, fiberdispersion, and resultant skew between client signal portions).

A client signal portion by itself, or a VWG, may be re-routed at a nodeto travel a different path. A re-routing of this sort is accomplished bycommunicating the client signal portion(s) to a multiplexing device,such as a band multiplexing module (“BMM”) shown in subsequent FIG. 8,which subsequently multiplexes optical signals and communicates them toa given node. Rerouting of a VWG in the present disclosure isaccomplished by switching in the electrical domain of a node and routinga client signal portion to a multiplexing module, whose function is tocombine carrier frequencies within a given carrier group fortransmission on a fiber medium, as shown in subsequent FIG. 8. If aclient signal portion is switched to a different multiplexing module,then the same carrier wavelengths may be utilized for both VWGs, as theywill not conflict on different multiplexing modules routing on differentfiber links. If client portion signals are communicated on the samecarrier wavelengths for different VWGs (e.g., on different paths) and ifthe client portion signals are to be combined or redistributed at asubsequent node, then any potential conflict of client signal portionson the same wavelength at that downstream node can be resolved byassigning appropriate non-conflicting wavelengths at the given node, asdirected by the controller.

In FIGS. 8-10, a novel switching function in a node of a network isillustrated in accordance with various aspects of the present invention.The switching function allows rerouting of a portion of information,such as a portion of a client signal, on a different wavelength,different fiber, and/or to a different node. Rerouting can be managed soas to provide for skew adjustment in order to provide better quality ofservice of the overall information transmitted over the network.

FIGS. 8-10 also illustrate an apparatus for implementing the skewadjustment within an exemplary node that also employs optical toelectrical to optical (“OEO”) conversion. Once in the electrical domain,client signals enjoy the benefits of digital signal processing,switching (channel and band allocation), and signal regeneration thatcorrespond to electronically implemented functions. However, the presentinvention is also well suited to performing functions off frequencytranslation/conversion for purposes of rerouting in the optical domain,e.g. using a PIC, PLC, or discrete electro-optical and optical devices.For example, a nonlinear process in semiconductor optical amplifiers(“SOAs”) or a highly nonlinear fiber could be utilized to satisfyfrequency translation/conversion needs. Additional detail on the design,function and architecture of the TxPIC, RxPIC and DON can be found inU.S. patent application Ser. Nos. 10/267,331, 10/267,212, and U.S. Pat.No. 7,116,851, all of which are incorporated by reference in theirentirety.

Referring in particular to FIG. 8, system 500 includes a transceivernode 502 coupled to receiver 530 and coupled to Node N6 (not shown) viaswitches 526A and 526B in accordance with various aspects of the presentinvention. Transceiver node 502 is coupled to one or more multiplexingmodules, such as band multiplex modules (“BMM”), each coupled todifferent nodes for space diversity routing. Transceiver node 502correlates to exemplary node N2 from FIGS. 7A-7D, and has a portion ofcommunication paths from the WDM signal outputs from TxPIC1 throughTxPIC8 coupled via BMM1 520 and fiber link 510 to a downstream receiver530 and its BMM 532 correlated to exemplary node N3 from FIGS. 7A-7D.

Transceiver node 502 is a multi-channel device with multiple DLM 503modules each of which contain an RxPIC and a TxPIC, a group of which arecoupled into a band MUX module (“BMM”) that multiplexes the range ofwavelengths (e.g., TxPIC1 λ₁ through TxPIC8 λ₃₂) into a WDM signal fortransmission on fiber link 510 to a downstream node. Inputs 508 and 509are coupled from upstream nodes in the communication network. Withineach DLM, electronic processing and switching blocks 522 and 523 provideoptions to manage the transmitted information in the electrical digitaldomain, including skew management functions, described in more detail insubsequent figures. While all the wavelengths processed by transceiver502 may be within in the C-band, this band may be divided between a redportion of the C-band, to represent lower wavelengths in the signalspectrum, and the blue portion of the C-band, to represent higherwavelengths in the signal spectrum. While the present embodimentconstrains the spectrum of wavelengths for transmission within theC-band, the present invention is well-suited to using any combinationand location of wavelengths such as utilizing multiple bands, e.g.,L-band, S-band, any other band or to utilizing divisions within a band,for communication path diversity.

In certain embodiments, two nodes may be coupled via multiple fibersthat can be selected for their different skew properties, such as theirdifferent dispersion properties between channels that will allowcarriers at different wavelengths to arrive at a downstream node atdifferent times. Transceiver node 502 has BMM2 521 coupled to node N3via switch 526A and 526B on either end of the multiple links 512 through516, which correlate, for example, to fiber F₁ 440 through fiber F_(N)444 of FIG. 7C, with different dispersion slopes K5 through K6,respectively. Switches 526A and 526B are any switch, that functions tocouple one of the multiple fibers to each node, such as by an external1xN mechanical switch, thermo-optic optical switch, ormicro-electrical-mechanical (“MEMs”) switch.

Referring now to FIGS. 9 and 10, a more detailed illustration of anexemplary transceiver digital line module (“DLM”) 503A is presented intwo parts, with FIG. 9 illustrating a receiver portion of the DLM, andFIG. 10 illustrating a transmitter portion of the DLM. DLM 503A in FIGS.9 and 10 correspond to DLM 503 block portion of the transceiver 502shown in FIG. 8. Output A from receiver portion of DLM is received asinput A at the transmitter portion of DLM.

Referring specifically to FIG. 9, a schematic of a receiver portion 600Aof a digital line module 503A is shown wherein the receiver has opticalskew compensation, with electronic skew measurement and skew buffer inaccordance with various aspects of the present invention. Receiverportion of DLM 503A has an optical domain 602 with customary componentssuch as 1:N DEMUX and an array of photodetectors (PDs) for λ₁ to λ_(N).

Certain embodiments provide coupling from the photodetectors to aprogrammable skew measurement device 622. The skew measurement device isenabled to capture skew measurements via a comparator (e.g., adifferential sense amplifier, and other digital signal processingtechniques) that correlates the output from a photodetector with apredetermined bit pattern. The bit pattern is replicated in a marker ofa test signal transmitted to the DLM 503A during a learning mode for thenetwork. This skew testing process is also referenced in process 1700 ofFIG. 6. Skew measurement device 622 has multiple instances ofcorrelation ability along with a local clock input for measuring thedifference in time from receipt of the marker for each of the multiplechannels λ₁ to λ_(N). Alternatively, programmable skew measurementdevice 622 may include the capability to perform a relative comparisonmeasurement between any two wavelengths at a given time for comparisontesting. This pattern can be repeated for different wavelengths, asdirected by local controller 620, in combination with a central networkcontroller.

Local controller 620 is coupled to skew measurement device 622, in thecontrol plane 632, to provide initiation signals for test mode,selection of wavelengths to measure, and reception of skew data. Localcontroller 620 in the current node is coupled via a unidirectional orbidirectional line 624 to other nodes in the network to share skew datameasurements, skew resource status, skew needs, and skew resourceallocation.

Besides providing skew measurement control, various nodes in theseembodiments of the invention provide an optional skew compensator 608for each channel in the optical domain 602 of the node and optional skewcompensator 610 in the electrical domain 604. Skew buffer 608 may be anyoptical device with delay properties, such as a ring resonator. Invarious embodiments, an optional skew compensator is provided for only aportion of the signal channels in the DLM 503A, such as on channels onwhich signals propagate at a higher rate per unit time, such as those onlower frequency channels. In other embodiments, optional skewcompensator has a bypass that is enabled via local controller 620 if noskew adjustment is needed. Lastly, in another embodiment, no opticalskew compensation is used because of higher cost, and sufficientcapability of skew adjustment via routing, and/or buffering in theelectrical domain.

Similar to optical skew buffer 608, optional electronic skew compensator610 may be any buffer medium, such as a first-in-first-out (“FIFO”)memory buffer, which delays the information on the given channel. Indifferent embodiments, optional electronic skew compensator 610 can beimplemented on all channels, or only on a fraction of the channels.Optional optical skew compensator 608 can be programmable to allow avariable amount of delay on the information transmitted thereon, with abypass to reduce any incidental propagation delay that the device mayexhibit even if no skew compensation is desired. Additionally, optionalelectronic skew compensator 610 may be located anywhere within theoptical networking system, including at transmitting nodes, receivingnodes and intermediary nodes. After the appropriate buffering in thereceiver, the electrical signals are communicated to switch 612, whichcan be any form of switch, such as cross-point switch, which enablesrerouting of information signals from one channel, or wavelength, toanother channel, or wavelength.

Referring specifically to FIG. 10, a schematic of a transmitter portionof a line module 600B is shown wherein the transmitter has optical skewcompensation and electronic skew buffer in accordance with variousaspects of the present invention. Transmitter receives the electricalsignals ‘A’ from the receiver of FIG. 9 or alternatively from a clientdata source 640, such as an add channel. Transmitter portion of DLM 503Aalso has electrical domain portion 642 and optical domain portion 644,with respective optional optical skew buffer 646, and optionalelectronic skew compensator 648. DLM 503A can utilize any combination ofthese delay devices in the transmitter and receiver as is applicable fora given design or application depending on the level of skew variationexhibited in the network. Optional buffers for FIGS. 9 and 10 arededicated, distributed in-line buffers in the present embodiment.However, in-line buffers can also be a centralized, shared memorybuffer, albeit with latency, cost, and flexibility tradeoffs.

Increased bandwidth that can be gained from co-routing signal portionsas described can be particularly advantageous in submarine opticalsystems, for example, in communicating between continents where thecommunication spans large bodies of water. FIG. 13 illustrates anexemplary multi-network, trans-oceanic optical system in which skewcompensation is realized at various locations along the signal pathincluding pre-compensation, intermediary compensation, andpost-compensation. In addition, the location and frequency of skewcompensation modules may depend on the number and diversity of theservice providers involved in the multi-network trans-oceanic system.

Various embodiments of the invention may be applied to submarine opticalsystems, some of which may be used as trans-oceanic optical networksthat connect terrestrial systems across a large body of water. Oneskilled in the art will recognize that the length in which an opticalsignal travels on these trans-oceanic systems presents diverseengineering issues including both dispersion and skew compensation.These issues are further complicated as the data rate of a client signalincreases and the total number of channels on which a signal istransmitted expands. One skilled in the art will recognize that thefollowing discussion, although described in relation to a trans-oceanicoptical system, may be applied to any type of networking system in whichskew and latency management are relevant, such as long-haul terrestrialoptical systems.

FIG. 11 illustrates generally a system in which both dispersion and skewpost-compensation are performed at a receiver side of an optical systemaccording to various embodiments of the invention. It is important tonote that the skew management functions and structures previouslydescribed above may be employed within this network at variouslocations. For example, the network may be installed, configured andmanaged at transmission nodes, intermediary nodes and/or receiver nodesto improve the differential latency between channels within the system.

On the transmission side of the system, a plurality of channels 2105 isoptically multiplexed, via multiplexer 2110, to generate a WDM signal.The WDM signal is communicated along the optical span having multipleoptical amplifiers or regenerators 2115 that keep the WDM signal powerwithin a preferred range. A coarse dispersion compensation module 2120is coupled to receive the WDM signal after having traversed all orsubstantially all of the optical span. The coarse dispersioncompensation module 2120 compensates for dispersion effects on the WDMsignal along the span, which causes signal degradation. In variousembodiments of the invention, the coarse dispersion compensation module2120 comprises dispersion compensating fiber or fibers that reduce thedispersive characteristics of the WDM signal. As the WDM travels throughthese dispersion compensating fiber(s), the shape of the signal isimproved resulting in a better signal-to-noise ratio.

One skilled in the art will recognize that various compensating systemsmay be realized with different types and combinations of dispersioncompensating fibers. Because the coarse dispersion compensation module2120 compensates for dispersion across the channels of the WDM signal(i.e., the WDM signal is multiplexed), targeting certain channels withinthe WDM signal for dispersion compensation is difficult. Accordingly,certain embodiments of the invention provide for additional finedispersion compensation at a channel granularity.

An optical demultiplexer 2125 separates the WDM signal into individualchannels, optical signal groups, or a combination thereof. A pluralityof fine dispersion compensation modules 2130 receive optical channels oroptical signal groups and further apply dispersion compensation thereon.In certain embodiments of the invention, each fine dispersioncompensation module 2130 is designed to compensate a certain channel orgroup of channels. Dispersion compensation fiber may be used within theplurality of fine dispersion compensation modules 2130.

The coarse dispersion compensation module 2120 and the fine dispersioncompensation module 2130 introduce additional latency within the WDMsignal. These latency effects become even more detrimental when theadded latency is not spread evenly across each of the channels. In suchsituations, this uneven addition of latency further increases the amountof skew between one or more of the channels resulting in a more complexand demanding reassembly procedure if not address prior thereto.

Each of the dispersion compensated channels is converted into theelectrical domain by a plurality of optical-to-electrical converters2135. These converters 2135 may include PIN diodes, photoavalanchediodes, or other converters known to one of skill the art. The resultingelectrical signals are provided to a plurality of skew compensatingmodules 2140 that adjust the differential latency between the channelsso that a signal, transmitted across at least two of the channels, maybe more efficiently rebuilt. This skew compensation may be achieved byeffectively introducing additional latency within one or more of thechannels by performing a post-buffering operation thereon. One skilledin the art will recognize that the buffer size in each of the skewcompensating modules 2140 may be adjusted to enable compensation of moreor less skew.

As previously discussed, skew is potentially introduced into a clientsignal as the channels within the WDM signal travel across the opticalspan and are processed within dispersion compensation modules (e.g.,2120, 2130). This skew may be compensated on the transmission side ofthe optical signal by pre-buffering one or more of the channels withinthe WDM signal, by buffering one or more of the channels within the WDMsignal at an intermediary node, or post-buffering one or more of thechannels at the skew compensating modules 2140, or any combinationthereof. According to various embodiments of the invention, the skewcompensating modules 2140 may also provide skew analysis functionalityin which skew across the channels is monitored. If the skew fallsoutside of a desired range, a skew compensating module 2140 may generatean alarm and/or dynamically re-allocate the channels to improve theskew. Furthermore, as detailed in FIG. 12, the skew compensating modules2140 may also be divided into coarse and fine skew compensating modules.

Although skew compensation has been described as being performed in theelectrical domain, one skilled in the art will recognize that skewcompensation may also be done in the optical domain. For example,additional latency may be added to one or more channels by using anoptical buffer, such as a fiber coil, to add this latency.

FIG. 12 illustrates a more detailed diagram of a receiver node within atrans-oceanic optical system according to various embodiments of theinvention. The node comprises a coarse dispersion compensation module2120 that compensates dispersion across the WDM signal as previouslydiscussed. An optical demultiplexer 2125 optically separates the WDMsignal into individual channels or optical signal groups, after which aplurality of fine dispersion compensation modules refine the dispersioncompensation at a finer granularity. After being converted intoelectrical channels by converters 2135, the skew across the electricalchannels is first coarsely adjusted and then finely adjusted.

In various embodiments of the invention, the electrical channels areprovided to a plurality of coarse skew compensating modules 2205. Thesemodules 2205 provide a coarse adjustment of differential latency betweenat least two of the electrical channels. This reduction of differentiallatency may be achieved by buffering one or more of the electricalchannels for a set period of time, which effectively reduces thecorresponding skew or differential latency between the electricalchannels. A plurality of fine skew compensating modules 2210 furtherrefines the skew compensation across certain channels. In certainembodiments of the invention, the plurality of fine skew compensatingmodules 2210 analyze certain skew characteristics remaining after thecoarse skew adjustment and further adjust the channels to furtherimprove the corresponding skew. One skilled in the art will recognizethat either or both of the coarse skew compensating modules 2210 and thefine skew compensating modules 2215 may be integrated with otherelectrical components within the node. For example, the fine skewcompensating modules 2215 may be integrated within an electricalmultiplexer 2215 that combines one or more electrical channels into aclient signal.

Further electrical components or modules may be provided within thesignal paths that analyze, modify or otherwise process these compensatedelectrical channels. These electrical components may or may not belocated between the coarse skew compensating modules 2210 and the fineskew compensating modules 2215.

Using the compensated electrical signals, a client signal 2220 istransmitted from the electrical multiplexer 2215 and is generated bycombining one or more of the electrical signals into a relatively higherdata rate signal. This combination of electrical signals is lessdemanding if there is little or no skew between its component electricalchannels.

FIG. 13 illustrates an exemplary multi-network, trans-oceanic opticalsystem in which skew compensation is realized at various locations alongthe signal path including pre-compensation, intermediary compensation,and post-compensation. In addition, the location and frequency of skewcompensation modules may depend on the number and diversity of theservice providers involved in the multi-network trans-oceanic system.

Referring to FIG. 13, a transmitting node 2305 transmits and/or receivesinformation from a first terrestrial network 2310. A first landing node2315 interfaces the first terrestrial network 2310 with a submarineoptical system 2320. A second landing node 2325 interfaces the submarineoptical system 2320 with a second terrestrial network 2330, which isconnected to a receiver node 2335. In this type of system, skewcompensation may be realized at various locations including thetransmitting node 2305, the first landing node 2315, the second landingnode 2325, and the receiver node 2335.

In various embodiments of the invention, pre-skew compensation isperformed exclusively on the transmitting node 2305, which compensatesfor skew across the first terrestrial network 2310, the submarineoptical system 2320, and the second terrestrial network 2330. Theseembodiments may be more typical if a service provider is using athird-party submarine optical system to inter-connect terrestrialnetworks and does not have control of the landing nodes of the submarineoptical system.

In other embodiments, skew compensation may be diversified throughoutthe system in which the first and/or second landing nodes 2315, 2325further comprise skew compensation modules. Such a diversificationallows a relatively lower amount of pre-compensation to be performed onthe transmitting node 2305 and a relatively lower amount ofpost-compensation to be performed on the receiver node 2335.Additionally, this diversification may also provide early fault or errordetection if skew becomes too large at some point within the system.

Network planning using skew considerations can also be utilized whenperforming a protection switching function. Protection switching occurswhen there is a break in a fiber, for example, when a fiber has beensevered by a backhoe or other piece of equipment. Protection switchingcan also occur when there is sufficient degradation in the signal toapproximate a broken fiber. When a protection switch event occurs, thecentralized or distributed controller determines an alternate path forthe signal that would otherwise be transported on the broken fiber.

FIG. 14 shows a block diagram of one embodiment of the present inventionperforming a protection switching operation. FIG. 14 illustrates asignal transporting between nodes 1400 and 1445 through network cloud1410. In the example shown, the signal is divided into four portionsthat are transported on four paths 1405. Two of the four paths 1415 aretransported on fibers that are intact. The other two channels 1425 aretransported on a fiber or fibers are have been compromised due tobreakage or signal loss for another reason. In one embodiment, paths1425 are re-routed on paths 1430 to node 1445 taking into considerationskew between paths 1415 and 1430. Paths 1415 and 1430 may be, but arenot required to be, on the same fiber. Any or all of the above describedskew determination and skew compensation techniques may be used inconjunction with this protection switching operation.

The networking system can detect a failure resulting in the need for aprotection switching operation using either conventional methods orusing the latency and skew determinations described above. In someembodiments using empirically determined skew data, the networkingsystem can detect a failure based on skew because the skew will growunacceptably large as a result of a failure. Consequently, the skew willbe outside of the skew constraints requiring the system to select analternate path or paths. Alternatively, conventional methods of failuredetection can be used. Once a failure has been detected the signal willbe rerouted to avoid the failure. When the signal is rerouted, thesystem can take into consideration the skew on the newly selected pathsand route the signal accordingly as described above.

FIGS. 15 and 16 are flowcharts illustrating processes, independent ofstructure for creating and updating look-up table 234 and for setting upnetwork paths taking into consideration skew. Referring to FIG. 15,latency can be predicted 1505, latency can be measured 1510 or acombination thereof, as described above in reference to FIG. 3. Thelatency can be used to determine skew 1515 as described above inreference to FIG. 3. Also, the latency values can be used to create alook-up table 1520 or can be stored in another format. Once the tablehas been created 1520, the latency values can be measured 1510 and thetable updated real time 1525. The table can be updated 1525 before thereis network traffic, while there is network traffic, or after networktraffic.

FIG. 16 illustrates a process for setting up network paths in accordancewith one embodiment. FIG. 16 assumes that the network will betransporting information as two or more signal portions over two or morecommunication paths. Therefore, the signal is divided 1605 into thosetwo more signal portions. At least two paths are analyzed as possiblecommunication paths for the two or more signal portions 1610. As part ofthe analysis the skew between the paths is determined 1615. The skew iscompared to a skew threshold 1620. If the skew is less than the skewthreshold, the network paths can be used 1625. The paths can also beanalyzed for a failure event 1640. If there has not been a failure thenthe paths can continue to be used and the process is complete 1645.

If the skew between the first two paths analyzed is greater than theskew threshold, then other paths will be analyzed if there are otherpaths available 1630. If there are no other paths available, then anotification is sent indicating that the signal cannot be sent as twosignal portions 1635. One skilled in the art will understand thatalthough the above process is described in regard to two communicationpaths, more than two paths can also be analyzed.

One skilled in the art will recognize that the above-described methodfor calculating latency across diverse paths may be applied to anynumber of paths greater than two. Additionally, the method may beapplied to any type of network including, but not limited to, submarine,trans-oceanic optical systems.

While the invention has been described in conjunction with severalspecific embodiments, it is evident to those skilled in the art thatmany further alternatives, modifications and variations will be apparentin light of the foregoing description. Thus, the invention describedherein is intended to embrace all such alternatives, modifications,applications, combinations, permutations, and variations as may fallwithin the spirit and scope of the appended claims.

1. A networking system for routing a signal over a plurality ofchannels, the system comprising: a transmission node that transmits thesignal on at least one of the plurality of channels; a first path thattransports a first portion of the signal on at least one channel in theplurality of channels; a second path that transports a second portion ofthe signal on at least one second channel in the plurality of channels;an initialization module, coupled within the networking system, thatdetermines the first path and the second path based at least in part ondetermined skew relative to the first path and the second path; and areceiver node that receives the first and second portions of the signaland reassembles the first and second portions of the signal.
 2. Thenetworking system of claim 1 wherein the initialization module predictsthe skew between the first communication path and the secondcommunication path.
 3. The networking system of claim 2 wherein theinitialization module predicts the skew by calculating the skew based onlatency of the first path and latency of the second path.
 4. Thenetworking system of claim 3 wherein a latency of the first and thesecond paths are stored in a look-up table.
 5. The networking system ofclaim 1 wherein a latency of the first and second paths is calculatedbased on transport characteristics of the networking system.
 6. Thenetworking system of claim 1 wherein the initialization module measuresthe skew between the first communication path and the secondcommunication path.
 7. The networking system of claim 6 wherein the skewis measured by measuring latency of the first communication path andlatency of the second communication path.
 8. The networking system ofclaim 1 wherein the initialization module predicts the skew between thefirst communication path and the second communication path and measuresthe skew between the first communication path and the secondcommunication path.
 9. The networking system of claim 1 wherein thereceiver node restores the signal using a buffer to compensate for theskew between the first and second communication paths.
 10. Thenetworking system of claim 1 wherein the initialization module takesinto consideration the skew by comparing the skew to a determined skewthreshold.
 11. The networking system of claim 10 further comprising anotification module, coupled within the networking system, to transmit anotification to the initialization module that the skew exceeds the skewthreshold.
 12. The networking system of claim 10 wherein theinitialization module seeks a third communication path when the skewexceeds the skew threshold.
 13. The networking system of claim 1 furthercomprising a protection switching module that performs a protectionswitch.
 14. The networking system of claim 1 wherein the first andsecond communication paths are at least partially over a degradableoptical medium.
 15. The networking system of claim 1 further comprisinga monitor module that monitors the skew between the first and secondpaths.
 16. The networking system of claim 1 wherein the initializationmodule is coupled within each node.
 17. The networking system of claim 1wherein the networking system is a submarine system.
 18. The networkingsystem of claim 1 wherein the networking system is a terrestrial system.19. The networking system of claim 1 wherein each channel is transportedon a carrier wavelength.
 20. The networking system of claim 1 whereinthe first and second communication paths are on different fibers.
 21. Atransceiver that determines a communication path, the transceivercomprising: an input on which a signal is received; a deinterleaver,coupled to the input, that partitions the signal into a first signalportion and a second signal portion; and a controller, coupled to thedeinterleaver, that determines a first communication path for the firstsignal portion and a second communication path for the second signalportion, the controller using skew between the first and secondcommunication paths to determine the first and the second communicationpaths.
 22. The transceiver of claim 21 where the determined first andsecond signal paths are based on paths determined having minimal skew.23. The transceiver of claim 21 wherein the controller predicts the skewbetween the first and second communication paths.
 24. The transceiver ofclaim 21 wherein the controller measures the skew between the first andsecond communication paths.
 25. The transceiver of claim 21 furthercomprising a notification module that notifies a user of the transceiverwhen the skew between the first and the second communication pathsexceeds a predetermined value.
 26. The transceiver of claim 21 whereinthe skew between the first and second communication is determined basedon a latency of the first communication path and a latency of the secondcommunication path.
 27. The transceiver of claim 26 wherein the latencyof the first communication path and the latency of the secondcommunication path are stored in a look-up table.
 28. The transceiver ofclaim 27 wherein the look-up table is dynamically updated.
 29. Thetransceiver of claim 21 further comprising a monitor module thatmonitors the skew between the first and second communication paths. 30.The transceiver of claim 21 wherein the transceiver is used inconjunction with a submarine networking system.
 31. A method for routinga signal over a plurality of channels, the method comprising:determining a first communication path for transmitting a first portionof the signal over at least one of the plurality of channels;determining a second communication path for transmitting a secondportion of the signal over at least one of the plurality of channels;and verifying that skew between the first communication path and thesecond communication path is acceptable.
 32. The method of claim 31further comprising transmitting the first portion of the signal on thefirst path and transmitting the second portion of the signal on thesecond path.
 33. The method of claim 31 wherein the first and secondcommunication paths are on the same transport optical medium.
 34. Themethod of claim 31 wherein the first and second communication paths areon different transport optical medium.
 35. The method of claim 31further comprising recombining the first and second signal portions at areceiver node.
 36. The method of claim 31 further comprising sending anotification regarding status of the determining the first and secondcommunication paths.
 37. The method of claim 31 further comprisingperforming a protection switching operation considering the skew betweenthe first and second communication paths.
 38. The method of claim 31further comprising storing a value related to skew in a tabular format.39. The method of claim 38 further comprising updating the value relatedto skew in a table.
 40. The method of claim 31 further comprisingmaintaining a value related to skew in a non-tabular format.
 41. Themethod of claim 31 further comprising performing a compensationoperation to compensate for skew between the first and secondcommunication paths.
 42. The method of claim 31 further comprisingseeking a third communication path when the skew exceeds the skewthreshold.
 43. A method for maintaining skew data, the methodcomprising: determining the latency data for a span; storing the latencydata and associated information identifying the span; updating the databased on further determination of latency data; calculating latency datafor a path from the latency data for a plurality of spans; anddetermining skew between a plurality of paths.
 44. The method of claim43 wherein the latency data for a span is determined empirically. 45.The method of claim 43 wherein the latency data for span is determinedtheoretically.